Pipe couplings are commercially available from a variety of manufacturing sources and are used extensively for underground pipe installations as in the distribution of natural gas. It is known in such couplings to employ gripping members of sorts in order to prevent in-service uncoupling of the joint by longitudinal drawbar forces imposed on the coupled pipe section. Reliability is essential in the construction of such couplings while cost of fabrication is likewise essential to the well being of the manufacturer for competing pricewise in the markets in which such couplings are sold.
With the advent of plastic pipe and tubing and their approval by regulating governmental agencies for natural gas transmission, the previous problem of pipe pull-out from a coupled joint increased many times over to on the order of ten fold as compared to metal pipe. One factor most contributing to this problem is the much greater linear coefficient of thermal expansion for plastic pipe as compared to steel pipe. By way of example, Aldyl "A" polyethylene piping marketed by Dupont has a linear coefficient of thermal expansion stated by the manufacturer to be: 9.times.1.sup.-5 in/in F.degree. as compared to 6.5.times.10.sup.-6 in/in F.degree. for steel pipe. Notwithstanding, government agencies regulating installation of such systems require that each joint sustain the longitudinal pullout or thrust forces caused by contraction or expansion of the piping or by anticipated external or internal loading to within prescribed limits. (Title 49 Section 192 "Transportation of Natural and Other Gas by Pipeline--Minimum Safety Regulations"--Fed. Vol. 35, No. 161).
While the degree of pipe restraint or lock up imposed on the coupled joint is critical, it is at least equally critical that adequate gasket compression be achieved about the coupled pipe in the course of coupling installation to insure a seal against leakage of line content.